Wet oxidation, also known as wet-phase oxidation, is an autocatalytic process that breaks down by oxidation organic or inorganic substances that are present in a liquid phase (aqueous solutions or suspensions) by using oxygen dissolved directly in said liquid phase.
The concentration of oxidizable species can be expressed as C.O.D. (cumulative oxygen demand), which expresses the amount of oxygen in mg/l or kg/m3 of water required for complete breakdown of the substances that are present.
The wet oxidation process is in itself exothermic, but to work in industrially acceptable times it is usually necessary to heat the wastewater to a temperature (generally 200-250° C.) at which the oxygen has a sufficiently high reactivity with respect to the sewage. When working at this temperature, however, it is necessary to apply intense pressures in order to keep the water in the liquid state.
At the same time, there is also a maximum temperature (generally 300-320° C., depending on the type of pollutants) that the wastewater cannot exceed. This limit arises from the fact that the maximum amount of heat that can be accumulated in the reaction water and in the gaseous phase that accompanies it (heat generated by the oxidation of the pollutants) is limited by the maximum operating temperature of the reactor and in any case cannot exceed 374° C., the temperature at which liquid water can no longer exist.
This factor is a particularly strong limitation when working with pure oxygen or oxygen-enriched air, since the limited amount of gaseous phase that is present in the reactor limits the evaporation of the water and the consequent absorption of the corresponding latent heat of evaporation. Accordingly, current processes, in order to operate safely, require the amount of pollutants treated per time unit and of introduced oxygen to not exceed certain levels set by the nature of the reaction and most of all by the practical method with which the wet oxidation is performed.
With temperatures of the input wastewater of approximately 200-270° C. and output temperatures of approximately 300-320° C., the maximum amount of oxygen that can be introduced (and therefore consumed) does not exceed 20-50 kg per cubic meter of water, whereas it would be instead highly desirable to be able to work at higher oxygen concentrations, so as to reduce the volumes of gas that pass through the reactor and have a higher amount of oxygen dissolved in the sewage (a quantity that depends on pO2).